1. Field of the Invention
The present invention relates to nondestructive inspection of metal stocks, and more specifically to a method of nondestructively inspecting metal pipe and bar stocks by an eddy current to detect a physical and/or metallurgical anomaly such as a flaw produced in the metal pipe and bar stocks.
2. Description of the Prior Art
Referring to FIG. 5, a prior nondestructive flaw inspection apparatus is illustratively shown in a cross sectional view, wherein an alternating magnetic field is applied to a sample to be inspected such as a metal pipe or a metal bar, etc., and a change in an eddy current produced in the sample is detected as any flaw being existent in the sample.
As shown in the figure, a sample 1 to be inspected such as a metal pipe or bar is transferred along its axis to the right at a low speed, and allowed to traverse the vicinity of a sensor 3 provided with an excitation/detection coil 2. The excitation/detection coil 2 is supplied with a radio frequency (RF) current from a RF power supply 6 and is allowed to generate a RF magnetic field on the sample 1 at a proper angle. This induces an eddy current along the sample 1. The sensor 3 is rotated by a rotary mechanism 4 around the sample 1 at a relatively high rotational speed together with the excitation/detection coil 2 to detect any change in the aforementioned eddy current flowing through the surface area of the sample 1, the change being caused by the existence of any flaw in the surface area of the sample 1. The change in the eddy current detected by the sensor 3 is fed to a signal processor 7.
Referring here to FIG. 6, orientations of surface flaws, which are assumed to be produced in the surface area of the sample 1, are depicted illustratively. In the prior nondestructive flaw inspection apparatus, the sensor 3 detects with ease a surface flaw L, which is oriented axially of the sample 1, because the sensor 3 scans the sample surface in a direction perpendicular to the surface flaw L, but it is difficult to detect a surface flaws when it is oriented as shown, particularly when it is short in length.
To solve the difficulty, U.S. Pat. No. 4,439,730 discloses a nondestructive inspection technique with use of a Hall element as a sensor, wherein two orthogonal steady magnetic fields are passed through an object to be inspected such as a pipe wall region, and a resulting composite signal from the object is detected and displayed. Hereby, the oriented flaw existing near a surface area of the object might be detected. However, the Hall element as a sensor is impracticable to the present situation where the excitation/detection coil is employed as a sensor to detect an eddy current produced in a sample.